body,
html {
  height: 100%;
}

.aa {
  width: 7rem;
  height: 100%;
  background-color: #cacaca;
  display: flex;
  flex-direction: column;
  /* 纵向排序 */
}

header {
  height: 1rem;
  background-color: #797979;
}


nav {
  flex: 1;
  color: #Fff;

}


footer {
  height: 1rem;
  background-color: #797979;
}